Devices made from high-temperature superconductors (HTS) operate at or near the boiling point (77K) of liquid nitrogen, a cheap coolant that is widely available and easy to store, handle, and transport. Superconducting quantum interference devices (SQUIDs) operating near 77K are desirable for a number of applications including non-destructive testing, geophysical surveys, medical imaging, and earthquake detection.
HTS materials are based on anisotropic layered crystalline structures. Colloquially speaking, each unit cell of HTS material has approximately the shape of a shoe box, a rectangular solid in which the three dimensions (a, b, and c) are different--a and b have similar values, and c is approximately three times greater. The superconducting properties in the a and b dimensions are similar, but the superconducting properties in the c dimension are dramatically worse. For reference, if the box is on its end on a table, the so-called "a-b plane" is parallel to the table, and the "c-axis" is perpendicular to the table. The a-b plane contains parallel sheets of copper and oxygen that have excellent superconducting properties. Along the c axis, perpendicular to the planes, the superconducting properties are poor.
In bulk polycrystalline samples of HTS materials, the critical current density (a measure of how much electrical current a superconductor can carry) is low, for two reasons. First, the grains of material are randomly oriented. As a result, in some grains, the current will flow along the dimension where superconducting properties are excellent, but in others, it must flow along the dimensions where superconducting properties are poor. Second, where "high-angle grain boundaries" exist--that is, where the adjacent grains are misoriented by more than 5.degree. and less than 90.degree.--the grain boundaries behave as "weak links," which have degraded superconducting properties.
It is possible to grow thin films of high temperature superconductors which contain no weak links. These films grow in a highly oriented manner, usually with the c axis perpendicular to the substrate and the copper-oxygen sheets parallel to the substrate. Such films are not truly single crystals--virtually all high-quality thin films contain many low-angle grain boundaries and twin boundaries. Fortunately, low-angle grain boundaries (where adjacent grains are misoriented by less than approximately 5.degree.) and twin boundaries (at which the grains are misoriented by approximately 90.degree.) do not degrade superconducting electrical transport properties. Therefore, highly-oriented films which contain no grain boundaries greater than approximately 5.degree. or less than approximately 90.degree. truly have no weak links, and they offer performance equivalent to a single crystal. Such thin films offer the potential for high-performance devices.